BAGHDAD -- In the early hours of an October morning, a security detail for Iraqi Vice President Adel Abdul Mahdi was racing down a mostly empty street in the capital's Karrada neighborhood when it came to a checkpoint set up by the Iraqi army. The soldier at the checkpoint yelled and motioned for the vehicles to stop, according to Iraqi and American security forces who described what happened. But the convoy, which consisted of four or five vehicles carrying 30 men, did not respond. The soldier fired a warning shot into the air.

That's when the trouble started, said Lt. Col. Robert M. Roth, commander of U.S. Army Task Force 4-46, which is helping train the new Iraqi army.

"The vice president's PSD grabbed the Iraqi army soldier and took him to the vice president's house," Roth said, using the abbreviation for private security details. "They said he tried to assassinate the vice president and refused to hand him over" to proper authorities.

The Iraqi Defense Ministry intervened and ultimately persuaded Mahdi's men to let the soldier go, 48 hours later. But Roth said the Oct. 16 incident highlighted the "huge problem" that Iraqi security details have become, even as U.S. forces attempt to turn over more security responsibilities to the Iraqis.

"I can't count the number of times on these two hands that a PSD of a minister or a delegate has tried to use his badge" to get into the heavily fortified Green Zone, without the official present, "because they didn't want to go through the normal line to get searched," he said.

Mahdi could not be reached for comment.

The Iraqi private security details are supposed to travel in vehicles marked with a number and with trained forces carrying credentials from the interior minister. But no single agency is responsible for enforcing the rules. The details routinely disregard Iraqi government forces, including soldiers and police, who patrol the streets.

"While some perform in a very professional manner, others cause fear in other motorists and pedestrians with their heavy-handed approach, including the use of warning shots," said Col. Edward Cardon, commander of the 4th Brigade Combat Team, 3rd Infantry Division in Baghdad. "Each ministry has its own PSD that is often loyal to a person rather than an official office. Hence there is no centralized training and no real accountability to anyone other than the person they protect. Their freedom and protection is directly related to the power of the person the PSD is protecting."

Residents of Washington are used to seeing Secret Service motorcades -- limousine caravans with dark-suited men trailing in SUVs. In Iraq, however, security convoys are something new and, for many, terrifying. The chaos caused by the insurgency makes it difficult to differentiate between the good guys and the bad guys. And there is no clear line of authority, which compounds the fear.

"The Americans pass by here, and we are more or less accustomed to their ways," said Ayad Taha, 31, a lawyer. "But the others are so unrecognizable. We don't know who they are, what nationality, whether they are legitimate or not. Really, they are scary."

Government leaders in Iraq say they use their own security details because they do not trust the official forces. Iraq's other vice president, Ghazi Yawar, whose security detail was not involved in the incident in Karrada, did not defend the actions of Mahdi's guards. But he said the threats against Iraqi government leaders made it reasonable for them to have special protection, just as other world leaders do.

Ayad Hashem, 40, a physician, said he was sympathetic to the government officials. "Yes, it is detestable to see those guys running around with guns, driving fast and at times shooting," he said. "But let's not forget we are still under occupation, really. I believe this stems from trying to imitate the Americans' behavior on the street. . . . The people involved are vulnerable and exposed to danger at any time."

Indeed, Iraqis have made the same complaints about U.S. military forces and Western private security details. The 4th Brigade's troops are under orders not to disrupt traffic unless absolutely necessary, but sometimes American convoys still push through the streets in an effort to avoid car bombers, roadside bombs and sniper fire.

Ahmad Karrar, 26, a traffic policeman in Karrada, said Iraqis would have to put up with the convoys. "There are many who are always buzzing by, every day," he said. "But we must accept this until we have stability and peace in this country."

But Mohammad Ali Akbar, 45, who owns a small stall on Karrada Street, said it was dangerous to allow the private details to operate in such a manner. "Their being on the street is terrorizing us," he said. "We never had such a phenomenon before -- or at least at this scale. Are they officially sanctioned? Nobody knows, and no one can tell us."

Baghdad's mayor, Sabir Isawi, said he was aware of the problem.

"We don't need PSDs," he said in a recent interview in his office. "We don't live in the Green Zone."

Isawi has been mayor since August, when armed men loyal to him stormed Baghdad's municipal building on the day the Iraqi government announced it had sacked the previous mayor.